In general, toner-fixing heat rollers in electrophotographic systems such as copiers and printers consist of heat rollers with heating means, and pressure rollers are disposed opposite the heat rollers. Recording paper to which a toner image has been transferred is passed between the two rollers so that heat and pressure are simultaneously applied to the paper in order to fix the toner image on the paper.
Such heat rollers have long comprised a photoemission-type heater tube such as a halogen lamp inserted in an aluminum or stainless steel pipe. Because the heat rollers use radiant heat, however, they are quite inefficient, and it can take anywhere from several tens of seconds to a few minutes to heat the heat rollers to the temperatures required for thermofixing (e.g. 160.degree. C.). This is especially problematic in photocopiers, where office efficiency is impaired by the long waits required before the machine is ready to resume copying after long, or even short, periods of non-use.
In recent years, copiers are hooked to other electronic equipment. In such systems, when a copier in the off or idle state receives a signal input, the system can be tied up for an extended period while waiting for the heat roller to reach operating temperature. This makes the copier a major obstacle to system speed. Regardless of how much faster the other equipment in the system becomes, it will be very difficult to achieve any significant gains in electrophotographic system speed without radical improvement in the area of toner fixing.
Further, a photoemission-type heater tube constantly generates light/heat, and it will heat the roller above the temperature that is set for it. To prevent this, heaters are controlled by an external circuit to turn them off and on when they reach a temperature near the desired setting. An undesirable side effect of this on/off control is heat roller temperature oscillation. If the amplitude of this oscillation (ripple) is large enough, it could, over time, result in toner thermofixing irregularity. Efforts to eliminate this problem causes the cost of the on/off control circuits to increase, and a small amount of irregularity still remained.
In addition, these photoemission-type heater tubes are basically glass lamps; and therefore, they are highly susceptible to damage from shock. Therefore, the heat rollers that contained these heater tubes requires very careful handling, and this reduces work efficiency. At the same time, these heat rollers consume a large amount of electrical power. Even when they are not in use, they need to be pre-heated, which is detrimental to energy conservation.
So as to mitigate these problems, methods that requires no photoemission-type heater tubes has been proposed. In this proposed heater roller, an electrical insulation layer made up of a highly heat-resistant organic resin such as polyimide is formed on the outer surface of a fixing heat roller (a cylinder made of a metal pipe); a resistive heating layer is provided on the outer surface of this electrical insulation layer; and finally, on the surface of this, a release layer of a material such as TEFLON (fluorocarbon resin) is formed (as disclosed in Japanese Patent Application Laid-Open (Kokai) Nos. S55-72390, S62-20038, and S63-158582). The idea behind this is that rapid heating of the entire heat roller can be achieved by heating the resistive heating layer by electric current.
Studies of this heat roller conducted by the inventor(s) of the present invention, however, revealed serious weaknesses that preclude its practical application. The materials used for the release layer and electrical insulation layer formed on the outer surface of the metal pipe are organic resins, and they are low in hardness. The recording paper is fed between the heat roller and a pressure roller under high pressure. Therefore, the release layer, which is exposed to the surface, and the electrical insulation layer lying just under it are acted on directly by external pressure. This makes them highly susceptible to damage.
Furthermore, a peeler finger for stripping the recording paper from the roller is provided in contact with the outer surface of the heat roller, and also a temperature sensing thermistor that is pressed against the outer surface is provided in contact with a certin amount of pressure. As a result, the outer surface of the heat roller is highly subject to damage and rapid wear. If the wear is allowed to progress to where the resistive heating layer is exposed, it can lead to unexpected problems such as electrical shorting. For the above reasons, this approach is impractical and therefore doomed to failure, and this technology has not, in fact, replaced photoemission-type heater tubes, which are still in use today. Moreover, there has been absolutely no solution to the temperature ripple problem.
Also, advances in digital technology have resulted in the introduction to the market of color copiers and of multifunction electrophotographic systems that integrate copier, printer, and fax functions in one machine. In particular, these multifunction systems are capable of handling papers of various sizes. For example a machine might process a sheet of B-5-size paper followed immediately by a sheet of A-4 or A-3 paper. After a sheet of B-5 paper passes over the heat roller, drawing heat from it, the portion of the roller that made contact with the paper will be much cooler than the rest of the roller. In other words, the temperature distribution across the surface of the roller will be extremely uneven, resulting in toner fixing irregularities for any larger-than-B5 sheets that follow a B-5 sheet through the machine. This problem is especially apparent in color copiers.
Japanese Patent Application Publication (Kokoku) No. H7-109531 proposes a system for providing uniform temperature distribution across the surface of the fixing heat roller by adjusting an electrical resistance heater so that a greater amount of heat would be applied to paper in the ready position before it reaches the heat roller. With a paper size mix ranging from B-5 through A-3, however, this system is not capable of keeping the temperature distribution constant through momentary changes in paper size.
To summarize, nothing in the past technology is capable of providing long service life, safety, fast heating, rapid correction of temperature irregularities to provide constant temperature, and temperature ripple control, all at the same time. There is still a need for a fresh approach to improvements in these areas of deficiency.